The quantity or density of surface antigens is important in the ability of a tumor cell to be recognized and killed by the immune system of the host. The way in which a cell regulates the quantity of surface antigens which it expresses is being studied in a model system of murine myelomas and lymphomas growing in tissue culture. Variants expressing reduced amounts of normal (Thy 1) or tumor-specific (Gross virus induced) surface antigens have been isolated. Serological, somatic genetic, and biochemical approaches are being used to study the basis of the variation observed.